Killed at the end of the Burning Crusade expansion, Kael'thas' soul was sent to Revendreth in the Shadowlands to work towards redemption for his sins in life. During his journey towards redemption, he fell right into Sire Denathrius' plans of funneling Revendreth's anima into The Maw, and the players must now save him from forever falling into damnation at the hands of the sinister Lord of Revendreth.

Sun King's Salvation is a two phase encounter in which the healers must restore the imprisoned soul of Kael'thas Sunstrider to full health. During phase one, players will need to deal with constant waves of adds that attempt to attack either the raid or Kael'thas Sunstrider directly. During phase two, Kael'thas Sunstrider becomes healing immune until players deal enough damage to a shade version of himself which batters the raid with powerful spells.

The Sun King's Salvation encounter is available after defeating Shriekwing in Castle Nathria. After defeating both the Sun King's Salvation and Artificer Xy'mox encounters, players unlock the Council of Blood encounter. 

Sun King's Salvation Quick Tips in Castle Nathria

Phase One General Strategy

• Heal Kael'thas Sunstrider as much as possible, whilst keeping mana efficiency in mind

• Players can use Soul Pedestals to sacrifice their own health to give to Kael'thas Sunstrider

Phase One Adds

• Rockbound Vanquisher - Kill them before their Concussive Smash is cast too many times and taunt off of each other when your Vanquished stacks drop.

• Bleakwing Assassin - Spread 8+ yards when marked with the Crimson Flurry and burst them down before they finish casting Return to Stone.

• Vile Occultist - Interrupt their Vulgar Brand or dispel the debuff it applies. Heal the Essence Font they drop for a burst of healing on Kael'thas Sunstrider.

• Pestering Fiend - Cannon fodder. Casually cleave them down, but don't be near them on Heroic when they die as they explode with Fragmentation!

• Soul Infuser - Crowd control and kill them before they reach Kael'thas Sunstrider and cast Soul Infusion. One healer should collect the Infuser's Orb for a big Infuser's Boon buff!

Phase Two

• Occur at 45% & 90% of Kael'thas Sunstrider remaining health.

• Kael'thas Sunstrider cannot be healed by players during this phase.

• Bring the Shade of Kael'thas mini-boss to the side of the room, away from his Reborn Phoenix adds.

• Ranged DPS should crowd control and kite the Reborn Phoenix adds around the outside of the room.

• The raid should stack behind the boss which is being faced away from them.

• Tanks should taunt off of each other when their Burning Remnants stacks expire (applied via Fiery Strike).

• A group of at least 5-6 players should soak the target marked for Ember Blast to help spread the damage of the ability.

• The player marked by Ember Blast should move 10 yards away from the raid into a low traffic area, as they will spawn a patch of fire underneath themselves when the spell ends.

• Bring the Shade of Kael'thas to 50% health to end the phase. Kill him the second time round to end the fight.

A full guide can be found on Wowhead: https://www.wowhead.com/guides/sun-kings-salvation-kaelthas-sunstrider-castle-nathria-raid-strategy-guide